Responses

The viewfinder cover prevents light leaking into the camera when it is not against your eye such as when taking a shot on a tripod. This light leak can affect the metering and the light in the final shot.
The part number that you are looking for is a Nikon DK5 and can be found via a google search or via your favourite online retailer.

Ron,
Almost anything that you can just hang over the back of the camera will work. Just about the only time I am needing this covered is when I am using the remote to trigger. In this case I usually hang my cap over the back and it blocks out the light. Maybe just a small swatch of black cloth will work. That little piece of plastic is more trouble than it is worth.
Being a country boy I always have a hat on.
phil b
benton, ky

Readily available on eBay in generic form (it's just a piece of plastic) from several sellers for under $3.00 shipped - just search for "Nikon DK-5". Boca Photo sells the "real deal" for $4.95, but it's currently back-ordered.

You could of course shoot in manual exposure mode for remote release shots, and then it doesn't matter if light enters the viewfinder. Or use your hat as Phil suggested for autoexposure modes. :-)